1. Field of the Invention
The present invention relates to a choke coil which is primarily used for eliminating noises generated by electronic equipment and the like.
2. Description of the Related Art
There are two modes for circulating noise. One is a normal mode (a differential mode) which circulates noise by generating a voltage difference between power supply lines. The other is a common mode which circulates noise by generating a voltage difference between the power supply lines and ground, but without a voltage difference between the power supply lines. The noise current direction of the normal mode is in the same direction as the current direction of the power supply. The noise current direction of the common mode follows a different loop than the current of the power supply. Choke coils are designed to reduce or eliminate these types of noise.
A common mode choke coil generally includes normal mode leakage inductance components, although at a low level. It is therefore effective also for normal mode noises. However, for normal mode noises at a high level, it has been necessary to use a separate normal mode choke coil to eliminate the noises.
In the case of a common mode choke coil having normal mode leakage inductance components at a relatively high level, leakage flux has sometimes adversely affected adjacent circuits. This has necessitated countermeasures such as a magnetic shield provided around a common mode choke coil.
Since it has not been possible for a single conventional choke coil to eliminate both common mode and normal mode noises sufficiently, in order to eliminate both common mode and normal made noises, it has been necessary to mount two choke coils, i.e., a common mode choke coil and a normal mode choke coil, on a printed circuit board or the like. This has resulted in a problem in that a large area is consumed on the printed circuit board or the like.
Further, a magnetic shield provided around a choke coil has led to an increase in the cost of the choke coil. It is therefore an object of the present invention to provide a choke coil having a sufficient noise eliminating effect against common mode and normal mode noises.